This invention relates to a control device for a glow plug which assists in the starting of a diesel engine.
A similar glow plug control device is disclosed in the U.S. Patent Application, Ser. No. 447,370, filed by the present inventor and others on Dec. 6, 1982.
It is well known in the art that it is necessary to heat the combustion chamber of a diesel engine in order to improve the starting characteristics of the engine, and glow plugs are used to so heat the combustion chamber.
Heretofore, it has taken about five to seven seconds to preheat the combustion chamber to a preset preheating temperature (about 900.degree. C.). However, it is rather difficult for an operator who has been familiar with gasoline engines to have to wait the preheating time, e.g. five to seven seconds, in starting the diesel engine. Accordingly, it is desirable to reduce the preheating time. This requirement may be satisfied by increasing the heating speed. However, in this case, the glow plug is quickly heated from a low temperature (about room temperature) to a high temperature (about 900.degree. C.). As a result, the temperature of the heat generating coil of the glow plug is greatly raised while the peripheral portion of the glow plug remains at low temperature. In order words, there is caused a large thermal gradient between the heat generating coil and the peripheral portion, with the result that thermal stress occurs in the glow plug. Accordingly, the heat generating element may be cracked or broken.
After the temperature of the glow plug has reached its predetermined value (900.degree. C. for instance), the voltage is lowered by a resistor provided between the glow plug and the power source, so that the large current to the glow plug is decreased and the glow plug is prevented from being melted.
Heretofore, this voltage dropping or lowering resistor has been a fixed resistor. However, the use of the fixed resistor suffers from the difficulty that since the temperature of the glow plug decreases after the interruption of the large current, it is difficult to make the glow plug sufficiently red hot, i.e., the starting characteristics of the engine are degraded.